I do believe a lot of mental disorders don't cause any extreme physical differences from that of someone without. I'm sure that someone who's depressed won't look as healthy, but overall I don't think they are very discernible. If you're talking about something like mental retardation then that's a different story I guess.
For physical attractiveness, I've read things on the symmetry of a face and attributes that signify health... like long, healthy hair, unblemished skin, estrogen levels, blah blah blah. The slight changes to the way a face is constructed can also subconsciously make us think a certain way about someone's character. It makes sense that we would be attracted to the healthiest of the bunch. But I do not think "beauty" is the only factor in the driving force of evolution, nature will still kill off the weakest overall according to our environment's fluctuations regardless if we find it attractive or not. That's because I don't think what's attractive to people is only ever what's best for survival, just very likely.